Edge softening with cycles material
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Cycles has a Bevel node (found in the add menu under Input>Bevel) that does exactly what you want. Simply plug the output of the Bevel node into the normal input on your shader/material and that's it. The example below shows a basic setup. The radius can be increased to make the corners look even more rounded.



Blender also has a Bevel modifier that will add the geometry very quickly as well. Depending on your model it may be an option too.